This invention relates to liquid crystal light valves, and, more particularly, to such a light valve having a fast response time and good spatial resolution.
A liquid crystal light valve is a device in which one beam of light is used to modulate the intensity and/or phase of another beam of light. The information contained in a "write-in" or input light beam is transformed into the corresponding information in a projection or output light beam. One application of the liquid crystal light valve is as an image amplifier or a light projector, wherein the write-in light beam has a weak intensity and the projection light beam has a strong intensity, with the information in the weak beam transformed into the corresponding information in the strong beam. Another application is a wavelength converter, where the write-in beam has one wavelength and the projection beam another wavelength. In yet another application, the write-in beam is ordinary incoherent light, and the projection beam is a coherent light beam produced by a laser, so that the information in the incoherent light beam is transformed into a coherent form of the same information (or the device may operate in the reverse fashion, with information in a coherent beam transformed to information in an incoherent beam).
Various types of liquid crystal light valves have been developed, including reflective and transmissive types. In a reflective liquid crystal light valve, a projection beam is directed against a mirror from an exterior light source. A thin layer, typically about 2-4 micrometers thick, of liquid crystal material is on the projection-beam side of the mirror. The projection beam passes through the liquid crystal layer twice, once before it reaches the mirror and again after it is reflected from the mirror. The projected beam is modulated by locally varying the transmission or phase relation of the liquid crystal according to the intensity of the write-in beam. For example, if at a particular location the write-in beam is of low light intensity, the liquid crystal is caused to have low transmittance so that not much of the projection beam is reflected. The liquid crystal can be made to operate in a generally proportional manner, so that the gradations in intensity of the write-in beam are transformed into corresponding gradations in intensity and phase of the projection beam.
The liquid crystal material is generally formed of elongated molecules that can be made to align in various directions under the influence of physical constraints and an applied electric field. The direction in which the molecules point, termed the director, determines the optical response of the liquid crystal to the light beam. In a typical liquid crystal light valve, the directors of the liquid crystal layer are initially oriented by the fabrication techniques to lie parallel to the surface of the mirror. The application of an electric field causes the directors to rotate toward an orientation perpendicular to the mirror.
In the liquid crystal light valve, the liquid crystal layer is on one side of the mirror. On the other side is a substrate structure that modulates an electric field applied to the liquid crystal responsive to the local intensity of the write-in beam. A number of different types of such light valve substrates have been devised, and one such structure, a MOS (metal-oxide-semiconductor) photosubstrate, is of particular significance to the present invention. The MOS photosubstrate structure, which has been known for over 10 years, includes a layer of dielectric material, typically silicon dioxide, adjacent the mirror, and a layer of high-resistivity semiconductor, typically silicon, contacting the dielectric material. An AC (alternating current) biasing voltage is applied across the liquid crystal layer/mirror/MOS photosubstrate. This voltage, in conjunction with the electrical current induced in the silicon by the incident write-in beam, produces the voltage variations that activate the liquid crystal to control the projection beam.
Some key operating parameters of a liquid crystal light valve are its spatial resolution, output modulation or contrast ratio (dynamic range), gray scale capability, wavefront distortion, and response time to changes in the write-in beam. Because of the physical characteristics of the liquid crystal layer, in existing MOS photosubstrate liquid crystal light valves there is a tradeoff between these parameters: to achieve a faster response time with a thin liquid crystal layer, the contrast ratio (dynamic range) is compromised.
Therefore, there is a need for an improved MOS photosubstrate liquid crystal light valve in which fast response time is achieved without sacrificing contrast ratio, and while achieving good spatial resolution and gray scale capability. The present invention fulfills this need, and further provides related advantages.